import com.sun.xml.internal.fastinfoset.util.CharArray;

import java.util.Scanner;

public class TestMain {
    public static  char[] fanzhaun(String str,int cmd) {
        int left = 0;
        int right = str.length() - 1;
        char[] arr = str.toCharArray();
        while(left < right) {
            char tmp = arr[left];
            arr[left] = arr[right];
            arr[right] = tmp;
            left++;
            right--;
        }
        return arr;
    }
    public static int yh(char[] arr1,char[] arr2) {
        int count = 0;
        for(int i = 0; i < arr1.length; i++) {
            if(arr1[i] !=  arr2[i]) {
                count++;
            }
        }
        return count;
    }
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        String str = scan.next();
        int per = 0;
        int cur = 0;
        char[] arr = str.toCharArray();
        while(cur < str.length()) {
            per += 1;
            cur += 2;
        }
        char[] arr1 = fanzhaun(str,per);
        System.out.println(yh(arr1,arr));
        scan.close();
    }
}
